The present invention relates to a reflow furnace for use in soldering of electronic components to a printed-circuit board using solder paste, and particularly to a reflow furnace in which soldering is carried out in an inert atmosphere.
Printed circuit boards which have been soldered are washed with a fluorine-containing solvent or chlorine-containing solvent to remove flux residuals in order to ensure reliability of the printed circuit boards. However, fluorine-containing solvents cause destruction of the ozone layer surrounding the Earth with an increase in uv radiation reaching the Earth's surface, which is thought to be harmful to human beings. On the other hand, chlorine-containing solvents cause pollution of underground water, making the water unsuitable as drinking water. Thus, strict regulations are applied to the use of such solvents so that an increase in production costs is inevitable.
Recently, a solder paste of the "low-residue" type has been used. The solder paste of this type contains a flux component comprising a reduced amount of solid matter such as rosin, and an activating agent (hereunder referred to as "mildly-activated rosin flux"), and the printed circuit board which is soldered using this type of solder paste does not require washing with a solvent after soldering. A mildly-activated rosin flux is a flux in which the content of rosin and an activating agent is reduced to as low a level as possible so that washing of the flux after soldering can be eliminated.
Generally, soldering with solder paste has been carried out in a reflow furnace in the presence of air, i. e., oxygen. However, soldering in air results in many soldering defects, e. g., formation of unsoldered portions and minute solder balls, which are caused by the presence of oxygen in the air. This is because the oxygen contained in the air easily oxidizes the soldering surface being heated in the furnace, and the oxidized surface adversely affects the spreading of molten solder.
It is known that if heating is carried out in the absence of oxygen, i.e., in an inert gas atmosphere such as one containing nitrogen gas, carbon dioxide gas, or argon gas, soldering can be carried out successfully without occurrence of any soldering defects. See Japanese Patent Application Laid-Open Specifications No. 64-83395/1989, No. 1-118369/1989, and No. 2-44185/1990.
In a solder reflow furnace containing an inert gas atmosphere (hereunder referred to as an "inert gas reflow furnace" or a "nitrogen reflow furnace"), the oxygen content is restricted to 3% by volume or less in order to diminish soldering defects when the soldering is carried out using solder paste, which is generally accepted as being superior with respect to solderbility. However, in the case of a solder paste employing an RMA flux (mildly-activated rosin flux), it is desirable to restrict the oxygen content to 1000 ppm (volume) or less, i.e., 0.1% by volume or less, since the low-residue paste solder is not so activated that it is rather difficult to remove an oxide film once formed on a soldering surface. As stated above, an oxidized soldering surface has an adverse effect on the spreading of molten solder and it also causes formation of minute solder balls from the oxidized solder alloy particles.
In the case of a low-residue type solder paste, the use of an inert gas reflow furnace is advantageous for further improving the spreading of molten solder alloy and markedly suppressing formation of minute solder balls.
Thus, in the case of an inert gas reflow furnace, the lower the oxygen content within the furnace, the more widely the molten solder alloy spreads and the less the formation of solder balls takes place.
Furthermore, in the case of conventional furnaces, when a large amount of nitrogen gas is introduced into the furnace, excess nitrogen gas flows out of the furnace through the inlet and outlet ports. It has been thought, therefore, that no air comes into the furnace. However, according to the experiences of the present inventors, it is inevitable that air comes into the furnace to increase the oxygen content to higher than 1000 ppm (volume) even if a large amount of nitrogen gas is supplied into the furnace. This tendency is marked for a reflow furnace of the circulating gas type.
Japanese Patent Application Laid-Open Specification No. 62-183960/1987 proposes the provision of dual shutters each at the outlet and inlet ports of a nitrogen gas reflow furnace so as to decrease the oxygen content in the atmosphere within the furnace.
However, a nitrogen gas reflow furnace with shutters installed at its entrances has many disadvantages. For example, it is necessary to provide a pair of dual shutters at both the outlet and inlet ports together with a waiting zone between the dual shutters and it is impossible to use a conveyor so as to carry printed circuit boards into and out of the furnace. Instead, a pusher must be used to transport printed circuit boards through the furnace. A continuous carrying system cannot be achieved using a pusher, and it takes time for each of the dual shutters to be opened one by one. After the first shutter is opened, the circuit boards to be soldered have to be kept waiting in the waiting zone until the second shutter is opened- This markedly decreases productivity. Furthermore, the provision of shutters and pushers makes the structure of the reflow furnace complicated, and it is rather difficult to synchronize the pusher with the shutter when printed circuit boards are taken into or taken out of the furnace.
However, in the conventional nitrogen gas reflow furnace which is free from shutters it is impossible to reduce the content of oxygen thoroughly even if an increased amount of nitrogen gas is introduced into the furnace.